feat(export): Obsidian-compatible Markdown vault as streamed ZIP

- GET /export/markdown streams the workspace as ZIP (no temp files):
  notes as .md with YAML front matter in the user's folder structure
  (shared notes and mounted folders included, empty folders kept)
- Clean Markdown: top-level headings/dividers/callouts/code as real blocks
  (their children move up a level), lists with tabs, todos as - [ ]
- Aufgaben/<list>.md in Obsidian Tasks format (due date, priority,
  subtasks, notes), Listen/<list>.md for shopping and other lists
- Safe, unique file names; download button in settings (mobile-safe)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -109,6 +109,7 @@ func (s *Server) Router() http.Handler {
 		r.Get("/api/v1/stats", s.handleStats)
 		r.Get("/api/v1/search", s.handleSearch)
 		r.Get("/api/v1/export", s.handleExport)
+		r.Get("/api/v1/export/markdown", s.handleExportMarkdown)
 		r.Get("/api/v1/themes", s.handleGetThemes)
 		r.Post("/api/v1/themes", s.handleSaveTheme)
 		r.Delete("/api/v1/themes/{id}", s.handleDeleteTheme)
@@ -2117,6 +2118,17 @@ func (s *Server) handleDeleteTheme(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]bool{"deleted": true})
 }
 
+// handleExportMarkdown streams an Obsidian-compatible ZIP of the user's workspace (no temp files).
+func (s *Server) handleExportMarkdown(w http.ResponseWriter, r *http.Request) {
+	userID := auth.GetUserID(r.Context())
+	w.Header().Set("Content-Type", "application/zip")
+	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="mindsift-vault-%s.zip"`, time.Now().Format("2006-01-02")))
+	if err := s.store.WriteMarkdownVault(userID, w); err != nil {
+		// Headers are already sent; the client gets a truncated archive
+		slog.Error("Markdown-Export fehlgeschlagen", "user_id", userID, "error", err)
+	}
+}
+
 func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
 	userID := auth.GetUserID(r.Context())
 	exportData, err := s.store.ExportUserData(userID)